Fiber optic pressure sensor

RELATED APPLICATION

0001This application is a continuation-in-part application of pending U.S. patent application Ser. No. 10/446,256 to Lagakos et al., entitled “Intensity Modulated Fiber Optic Pressure Sensor” and filed on May 28, 2003, which pending patent application is incorporated herein by reference in its entirety. The present patent application claims benefit of priority to the pending U.S. patent application Ser. No. 10/446,256.

TECHNICAL FIELD

0002The present invention relates generally to pressure sensors, and more particularly to fiber optic pressure sensors.

BACKGROUND ART

0003Active sound control systems often require specialized sensors, such as microphones to measure variations in atmospheric pressure. Commercially available electrical microphones, for example, detect pressure applied on their diaphragm by monitoring small changes in the capacitance measured between the diaphragm and a backplate placed closely to the diaphragm. Such electrical microphones are typically of two types: the electret microphone, which is biased by a built-in charge, and the condenser microphone, which is biased by an external voltage source. Electret microphones suffer from long-term instability due to their charge decay, whereas condenser microphones need a substantial external bias voltage, which in some applications is not desirable.

0004Both types of electrical microphones are susceptible to electromagnetic interference (“EMI”), which is strong at low frequencies. For example, in some active control systems, the presence of high voltages required for the controllers introduces substantial EMI, which increases the noise of the electrical microphones. To minimize EMI, electrical microphones often have a preamplifier attached to the microphone head. Lead wires close to the microphone head introduce extra capacitance, which can degrade the capacitance signal of the microphone. The lead capacitance problem is minimized by placing the preamplifier next to the microphone head. However, having the preamplifier next to the microphone head, even though minimizing EMI and lead capacitance noise, makes the microphone heavy, large, and expensive.

0005Examples of fiber optic microphones include those disclosed in “Theoretical and experimental study of a fiber optic microphone,” Hu et al., J. Acoust. Soc. Am. 91 (5), May 1992, and “High-temperature fiber-optic lever microphone,” Zuckerwar et al, J. Acoust. Soc. Am. 97 (6), June 1995. Hu et al. disclose a fiber optic microphone that includes a metallized Mylar membrane that is stretched and cemented to a fiber optic microphone case. Zuckerwar et al. disclose a fiber optic microphone cartridge that secures a membrane made of nickel 200 foil. Typical fiber optic microphones have a diaphragm diameter of around 0.55 cm, a diaphragm thickness of around 12.5 μm, and a minimum detectable pressure of around 5 mPa/Hz^1/2. Fiber optic microphones have typically not replaced electrical microphones in many applications because their performance and/or cost-effectiveness have been inadequate.

DISCLOSURE OF THE INVENTION

0006In an embodiment of the invention, an apparatus includes an optical fiber for transmitting light. The optical fiber has an axis. The fiber optic pressure sensor also includes an etched diaphragm perpendicular to the axis. The diaphragm includes an operative side located at a distance from the optical fiber sufficient to reflect a portion of the transmitted light into the optical fiber. The etched diaphragm includes a semiconductor, a dielectric, and/or a metal.

0007Optionally, the semiconductor diaphragm includes CdTe, CdZnTe, InP, InSb, GaAs, GaN, GaP, GaSb, Ge, Si, SiGe, Sn, ZnSe, ZnS, an oxide thereof, and/or a diamond. Optionally, the semiconductor diaphragm has a diaphragm thickness between 0.1 μm and 2 μm, and/or a cross-sectional width between 0.1 mm and 5 mm.

0008Optionally, the semiconductor diaphragm includes a reflective layer for reflecting the portion of the light into the optical fiber. For example, the reflective layer includes aluminum, beryllium, chromium, copper, gold, molybdenum, nickel, platinum, rhodium, silver, tungsten, and/or an alloy thereof.

0009Optionally, the semiconductor diaphragm includes an exposed side opposite to said operative side. A protective layer is located adjacent to said exposed side. For example, the protective layer includes a semiconductor, an oxide ceramic, and/or a non-oxide ceramic. For example, the semiconductor includes CdTe, CdZnTe, InP, InSb, GaAs, GaN, GaP, GaSb, Ge, Si, SiGe, Sn, ZnSe, ZnS, an oxide thereof, and/or a diamond. For example, the oxide ceramic includes MgO, TiO₂, SiO₂, MnO₂, Cr₂O₃, Fe₂O₃, and/or Al₂O₃. For example, the non-oxide ceramic includes BN and/or TiB₂. For example, protective layer defines at least one hole.

0010Optionally, a housing surrounds the optical fiber. A head encloses a portion of the housing. The housing and the head defines a gas reservoir. The head includes a spacer between the optical fiber and the diaphragm. The spacer defines one or more apertures communicating with the gas reservoir. For example, the spacer includes a spacer thickness. The spacer thickness is about equal to the distance. For example, the distance is between 170 μm and 270 μm, or between 700 μm and 1100 μm.

0011Optionally, a light source communicates with the optical fiber for generating the light transmitted through the optical fiber. For example, the light source includes a light emitting diode, an amplified spontaneous emission light source, a gas laser, a dye laser, and/or a laser diode.

0012Optionally, a photodetector communicates with the optical fiber to receive the portion of reflected light. For example, the photodetector includes a PIN detector, an avalanche photodiode, a photomultiplier tube, and/or a metal-semiconductor-metal detector.

0013In another embodiment of the invention, an apparatus includes a transmitting optical fiber for transmitting light. The transmitting optical fiber includes a transmitting end having a transmitting axis. The apparatus also includes one or more receiving optical fibers each having a receiving end having a receiving axis. The apparatus also includes a semiconductor diaphragm perpendicular to the transmitting axis and the receiving axis, and located at a distance from the optical fiber sufficient to reflect a portion of the light from the transmitting end to the receiving end.

0014Optionally, the one or more receiving optical fibers includes a plurality of receiving optical fibers. For example, the plurality of receiving optical fibers includes three, four, five, six, seven, and/or eight receiving optical fibers.

0015In another embodiment of the invention, an apparatus includes a transmitting optical fiber for transmitting light. The transmitting optical fiber includes a transmitting end having a transmitting axis. The apparatus also includes one or more receiving optical fibers, each comprising a receiving end having a receiving axis parallel to the transmitting axis. The apparatus also includes a semiconductor diaphragm located at a distance from the optical fiber sufficient to reflect a portion of the light from the transmitting end to the receiving end.

0016In another embodiment of the invention, an apparatus includes an optical fiber for transmitting light. The optical fiber includes an axis. The apparatus also includes an etched diaphragm perpendicular to the axis. The diaphragm includes an operative side located at a distance from the optical fiber sufficient to reflect a portion of the transmitted light into the optical fiber. The etched diaphragm includes a semiconductor, a dielectric, and/or a metal.

0017Optionally, the etched diaphragm includes a reactive ion etched diaphragm, a chemical etched diaphragm, and/or a mechanical etched diaphragm. For example, the etched diaphragm includes a diaphragm thickness between 0.1 μm and 2 μm, and/or a cross-sectional width between 0.1 mm and 5 mm. For example, the semiconductor includes CdTe, CdZnTe, InP, InSb, GaAs, GaN, GaP, GaSb, Ge, Si, SiGe, Sn, ZnSe, ZnS, an oxide thereof, and/or a diamond. For example, the dielectric includes SiO₂, SiN, SiC, and/or Si₃N₄. For example, the metal includes aluminum, beryllium, chromium, copper, gold, molybdenum, nickel, platinum, rhodium, silver, tungsten, and/or an alloy thereof.

0018In another embodiment of the invention, an apparatus includes a transmitting optical fiber for transmitting light. The transmitting optical fiber includes a transmitting end having a transmitting axis. The apparatus also includes one or more receiving optical fibers, each comprising a receiving end having a receiving axis. The apparatus also includes an etched diaphragm perpendicular to the transmitting axis and the receiving axis, and located at a distance from the optical fiber sufficient to reflect a portion of the light from the transmitting end to the receiving end. The etched diaphragm includes a semiconductor, a dielectric, and/or a metal.

0019Optionally, the etched diaphragm includes a reactive ion etched diaphragm, a chemical etched diaphragm, and/or a mechanical etched diaphragm. The etched diaphragm includes a diaphragm thickness between 0.1 μm and 2 μm, and/or a cross-sectional width between 0.1 mm and 5 mm. For example, the semiconductor includes CdTe, CdZnTe, InP, InSb, GaAs, GaN, GaP, GaSb, Ge, Si, SiGe, Sn, ZnSe, ZnS, an oxide thereof, and/or a diamond. For example, the dielectric comprises SiO₂, SiN, SiC, and/or Si₃N₄. For example, the metal includes aluminum, beryllium, chromium, copper, gold, molybdenum, nickel, platinum, rhodium, silver, tungsten, and/or an alloy thereof.

0020In another embodiment of the invention, an apparatus includes a transmitting optical fiber for transmitting light. The transmitting optical fiber includes a transmitting end having a transmitting axis. The apparatus also includes one or more receiving optical fibers, each comprising a receiving end having a receiving axis parallel to the transmitting axis. The apparatus also includes an etched diaphragm located at a distance from the optical fiber sufficient to reflect a portion of the light from the transmitting end to the receiving end. The etched diaphragm includes a semiconductor, a dielectric, and/or a metal.

0021Optionally, the etched diaphragm includes a reactive ion etched diaphragm, a chemical etched diaphragm, and/or a mechanical etched diaphragm. For example, the etched diaphragm comprises at least one of a diaphragm thickness between 0.1 μm and 2 μm, and a cross-sectional width between 0.1 mm and 5 mm. For example, the semiconductor includes CdTe, CdZnTe, InP, InSb, GaAs, GaN, GaP, GaSb, Ge, Si, SiGe, Sn, ZnSe, ZnS, an oxide thereof, and/or a diamond. For example, the dielectric includes SiO₂, SiN, SiC, and/or Si₃N₄. For example, the metal includes aluminum, beryllium, chromium, copper, gold, molybdenum, nickel, platinum, rhodium, silver, tungsten, and/or an alloy thereof.

0022Advantageously, an embodiment of the instant invention has EMI immunity, as its sensor output is optical and its electro-optical components can be placed remote from its diaphragm.

0023Advantageously, an embodiment of the instant invention does not require a preamplifier, reducing its weight and size than might otherwise be possible.

0024Advantageously, an embodiment of the instant invention has, for example, a diaphragm diameter of around 0.16 cm, a diaphragm thickness of around 1.5 μm, and a minimum detectable pressure of around 3 mPa/Hz^1/2. Total sensor size for an embodiment of a fiber optic sensor according to the instant invention, for example, is 2.8 mm o.d. and 0.5″ long.

0025Fiber optic sensors according the instant invention benefit from potentially high bandwidth, high security, and operation in hostile environments, such as environments with high electric fields and/or chemically corrosive and explosive environments. They can be used singly or in arrays, and are thus suitable for a variety of applications, such as structural or mechanical integrity applications, and medical applications. By way of example, an embodiment of a fiber optic sensor according to the instant invention, if placed near to a motor would detect changes in frequencies of sounds from the motor, which might indicate an impending structural flaw or undesirable wear. As another example, the potentially slim design of an embodiment of a fiber optic sensor according to the instant invention makes it ideal for minimally invasive medical diagnostic procedures.

BRIEF DESCRIPTION OF THE DRAWINGS

0026<figref idref="DRAWINGS">FIG. 1</figref><i>a </i>is a cross-sectional view of an embodiment of the invention.

0027<figref idref="DRAWINGS">FIG. 1</figref><i>b </i>is a cross-sectional view taken through line <b>1</b><i>b</i>—<b>1</b><i>b </i>of <figref idref="DRAWINGS">FIG. 1</figref><i>a. </i>

0028<figref idref="DRAWINGS">FIG. 1</figref><i>c </i>is a cross-sectional view taken through line <b>1</b><i>c</i>—<b>1</b><i>c </i>of <figref idref="DRAWINGS">FIG. 1</figref><i>a. </i>

0029<figref idref="DRAWINGS">FIG. 2</figref><i>a </i>is a cross-sectional view of an embodiment of the invention.

0030<figref idref="DRAWINGS">FIG. 2</figref><i>b </i>is a cross-sectional view taken through line <b>2</b><i>b</i>—<b>2</b><i>b </i>of <figref idref="DRAWINGS">FIG. 2</figref><i>a. </i>

0031<figref idref="DRAWINGS">FIG. 3</figref><i>a </i>is a cross-sectional view of an embodiment of the invention.

0032<figref idref="DRAWINGS">FIG. 3</figref><i>b </i>is a cross-sectional view taken through line <b>4</b><i>b</i>—<b>4</b><i>b </i>of <figref idref="DRAWINGS">FIG. 3</figref><i>a. </i>

0033<figref idref="DRAWINGS">FIG. 4</figref><i>a </i>is a cross-sectional view of an embodiment of the invention.

0034<figref idref="DRAWINGS">FIG. 4</figref><i>b </i>is a cross-sectional view taken through line <b>4</b><i>b</i>—<b>4</b><i>b </i>of <figref idref="DRAWINGS">FIG. 4</figref><i>a. </i>

0035<figref idref="DRAWINGS">FIG. 4</figref><i>c </i>is a cross-sectional view taken through line <b>4</b><i>c</i>—<b>4</b><i>c </i>of <figref idref="DRAWINGS">FIG. 4</figref><i>a. </i>

0036<figref idref="DRAWINGS">FIG. 4</figref><i>d </i>is a cross-sectional view taken through line <b>4</b><i>d</i>—<b>4</b><i>d </i>of <figref idref="DRAWINGS">FIG. 4</figref><i>a. </i>

0037<figref idref="DRAWINGS">FIG. 5</figref> is graph plotting the power of the reflected light as a function of probe-mirror distance for an embodiment of a 1 fiber probe according to the instant invention.

0038<figref idref="DRAWINGS">FIG. 6</figref> is a graph plotting the ac displacement sensitivity as a function of probe-mirror distance for an embodiment of a 1 fiber probe according to the instant invention.

0039<figref idref="DRAWINGS">FIG. 7</figref> is a graph plotting the power of the reflected light as a function of probe-mirror distance for an embodiment of a 7 fiber probe according to the instant invention.

0040<figref idref="DRAWINGS">FIG. 8</figref> is a graph plotting the ac displacement sensitivity as a function of probe-mirror distance for an embodiment of a 7 fiber probe according to the instant invention.

BEST MODES OF CARRYING OUT THE INVENTION

0041A reflection-type intensity modulated fiber optic sensor <b>5</b> according to the instant invention which can detect pressure is described by way of example in <figref idref="DRAWINGS">FIGS. 1</figref><i>a</i>, <b>1</b><i>b</i>, <b>1</b><i>c </i>as follows. For the purpose of this discussion, such a fiber optic sensor is called a 1 fiber probe. An optical fiber <b>10</b> is perpendicularly positioned at a distance from an etched diaphragm <b>20</b> sufficient to reflect at least a portion of light passing through and exiting from the optical fiber <b>10</b> back into the optical fiber <b>10</b>. The optical fiber <b>10</b> is a single mode or a multi-mode optical fiber, depending at least in part on cost constraints or performance requirements of the fiber optic sensor <b>5</b>. Upon a change in the atmospheric pressure, pressure waves cause the etched diaphragm <b>20</b> to distort causing a change in the amount of light reflected by an operative side of the etched diaphragm <b>20</b> back into the optical fiber <b>10</b>. The intensity of the light coupled into the optical fiber <b>10</b> modulates in relation to the intensity of the pressure wave causing the etched diaphragm <b>20</b> to distort.

0042The etched diaphragm <b>20</b> includes a material amenable to a planar processing step of etching. Examples of suitable types of etching depend on the diaphragm material and include reactive ion etching, chemical etching, and mechanical etching. Acceptable diaphragm materials include, for example, semiconductors, dielectrics, and metals. Acceptable semiconductors include, for example, CdTe, CdZnTe, InP, InSb, GaAs, GaN, GaP, GaSb, Ge, Si, SiGe, Sn, ZnSe, ZnS, an oxide of any of these semiconductor materials, and/or a diamond. Acceptable dielectrics include, for example, SiO₂, SiN, SiC, and/or Si₃N₄. Acceptable metals include, for example, aluminum, beryllium, chromium, copper, gold, molybdenum, nickel, platinum, rhodium, silver, tungsten, and/or an alloy of one or more of these metals. Optionally, the etched diaphragm <b>20</b> has a diaphragm thickness and a cross-sectional width that depend, for example, on characteristics of the atmospheric pressure changes to be detected and/or robustness of the fiber optic sensor. For example, the etched diaphragm <b>20</b> has a diaphragm thickness between 0.1 μm and 2 μm. For example, the etched diaphragm <b>20</b> has a cross-sectional width between 0.1 mm and 5 mm.

0043Optionally, a reflective layer <b>30</b> is located on an operative side of the etched diaphragm <b>20</b> for reflecting at least a portion of the light back into the optical fiber <b>10</b>. The reflective layer <b>30</b> includes, for example, a reflective metal such as aluminum, beryllium, chromium, copper, gold, molybdenum, nickel, platinum, rhodium, silver, tungsten, and/or an alloy of any of these reflective metals.

0044A preferred distance between the tip of the optical fiber <b>10</b> and either the etched diaphragm <b>20</b> or the reflective layer <b>30</b> depends on the physical characteristics of the material from which the light is being reflected and/or characteristics of the light itself. For example, for an etched diaphragm <b>20</b> made of silicon, such preferred distances are between 170 μm and 270 μm, and between 700 μm and 1100 μm.

0045Optionally, a protective layer <b>40</b> is located on an exposed side of the etched diaphragm <b>20</b>. The protective layer <b>40</b> helps maintain the structural integrity of the etched diaphragm <b>20</b> during use of the fiber optic sensor <b>5</b>. Optionally, the protective layer <b>40</b> includes one or more holes, exposing the etched diaphragm <b>20</b> to the environment from which the atmospheric pressure changes are to be detected. The protective layer <b>40</b> includes a material that can withstand the environmental use of the fiber optic sensor while protecting the etched diaphragm. For example, the protective layer <b>40</b> includes a semiconductor, an oxide ceramic, and/or a non-oxide ceramic. The semiconductor includes, for example, CdTe, CdZnTe, InP, InSb, GaAs, GaN, GaP, GaSb, Ge, Si, SiGe, Sn, ZnSe, ZnS, an oxide of one of these semiconductor materials, and/or a diamond. The oxide ceramic includes, for example, MgO, TiO₂, SiO₂, MnO₂, Cr₂O₃, Fe₂O₃, and/or Al₂O₃. The non-oxide ceramic comprises, for example, BN and/or TiB₂.

0046Optional housing <b>45</b>, connected directly or indirectly to the etched diaphragm <b>20</b>, surrounds the optical fiber <b>10</b>. For example, the housing <b>45</b> includes stainless steel tubing to protect the optical fiber <b>10</b>. Tubing having other protective materials are also suitable.

0047An optional head <b>50</b> surrounding an end of the optical fiber <b>10</b> is shown, by way of example, in <figref idref="DRAWINGS">FIGS. 2</figref><i>a </i>and <b>2</b><i>b</i>. The head <b>50</b> optionally includes one or more spacers <b>60</b> between the tip of the optical fiber <b>10</b> and either the etched diaphragm <b>20</b> or the reflective layer <b>30</b>. The thickness of the spacers <b>60</b> is selected, for example, to fix a distance between the end of the optical fiber <b>10</b> and the etched diaphragm <b>20</b>. Such a distance is, for example, between 170 μm and 270 μm, or between 700 μm and 1100 μm for an etched diaphragm <b>20</b> made at least in part of silicon. The head <b>50</b> optionally includes a cavity such that the head <b>50</b> and housing <b>45</b> define a gas reservoir <b>55</b>. Also, optionally, there is a gas space <b>65</b> between the tip of the optical fiber <b>10</b> and either the etched diaphragm <b>20</b> or the reflective layer <b>30</b>, the gas space <b>65</b> being at least partially bounded by the one or more spacers <b>60</b>. Optionally, the one or more spacers <b>60</b> define one or more apertures, which permit communication between the gas space and the gas reservoir. The gas, for example, is air. The communication between the gas space <b>65</b> and the gas reservoir <b>55</b>, for example, reduces gas pressure build-up in the gas space upon distortion of the etched diaphragm <b>20</b>.

0048The light is generated by an optional light source (not shown), which is optically connected directly, or indirectly via one or more lens, filters, or other beam-splitters, to the optical fiber <b>10</b>. The selection of the light source depends at least in part on cost constraints or performance requirements of the fiber optic sensor. Optionally, the light source generates coherent light. For example, such a light source includes one or more laser diodes (“LD”), gas lasers, or dye lasers. Alternatively, the light source generates incoherent light. For example, such a light source includes one or more light emitting diodes (“LED”) or amplified spontaneous emission (“ASE”) light sources.

0049An optional photodetector (not shown) is optically connected directly, or indirectly via one or more lens, filters, or other beam-splitters, to the optical fiber <b>10</b> to detect light reflected from the etched diaphragm <b>20</b>. The photodetector includes, for example, one or more PIN detectors, avalanche photodiode, photomultiplier tubes, and metal-semiconductor-metal (“MSM”) detectors. <figref idref="DRAWINGS">FIGS. 3</figref><i>a </i>and <b>3</b><i>b </i>show, by way of example, another embodiment of the invention. For brevity, only the differing characteristics of the embodiment of <figref idref="DRAWINGS">FIGS. 3</figref><i>a </i>and <b>3</b><i>b </i>relative to that of <figref idref="DRAWINGS">FIGS. 1</figref><i>a</i>, <b>1</b><i>b</i>, and <b>1</b><i>c </i>will be discussed below. A light source (not shown) is optically connected directly, or indirectly via one or more lens, filters, or other beam-splitters, to at least one transmitting optical fiber <b>70</b>. At least a portion of the light reflects from the etched diaphragm <b>20</b> into one or more receiving optical fibers <b>80</b>. The axes of the ends of the transmitting and receiving optical fibers <b>70</b>, <b>80</b> closest to the etched diaphragm <b>20</b> are perpendicular to the etched diaphragm <b>20</b>. The axes of the transmitting and receiving optical fibers <b>70</b>, <b>80</b> are substantially parallel. Optionally, the surfaces of the transmitting and receiving optical fibers <b>70</b>, <b>80</b> are abutting. Additionally, it is understood that the receiving optical fibers <b>80</b> may entwine around the one or more transmitting optical fibers <b>70</b> such that the distance between the surface of each receiving optical fiber <b>80</b> and the one or more transmitting optical fibers <b>70</b> is constant, although technically the axes thereof may not be exactly parallel. These characteristics, for example, help minimize the cross-sectional width of the fiber optic sensor according to the instant invention. A photodetector is optically connected directly, or indirectly via one or more lens, filters, or other beam-splitters, to the one or more receiving optical fibers <b>80</b> to detect the reflected light. As the number of receiving optical fibers <b>80</b> is increased, a greater percentage of the reflected light is detected. For example, the number of receiving optical fibers <b>80</b> is between three and eight.

0050<figref idref="DRAWINGS">FIG. 3</figref><i>a </i>also shows an optional supporting structure <b>75</b> for supporting the etched diaphragm <b>20</b>. The supporting structure <b>75</b> is attached to the diaphragm <b>20</b> and interposed between the diaphragm <b>20</b> and the housing <b>45</b>. For instance, the supporting structure includes a substrate on which the diaphragm <b>20</b> is formed. The substrate includes, for example, a semiconductor, a dielectric, and/or a metal. The thickness of the supporting structure <b>75</b> is, for example, one, two, three, or four orders of magnitude greater than the thickness of the diaphragm <b>20</b>. For example, the supporting structure <b>75</b> includes an annular or other conforming shape around the periphery of the diaphragm <b>20</b>. By way of illustration, because of the thinness of the diaphragm <b>20</b>, the diaphragm itself may be difficult to manipulate easily without inadvertent breakage. Such a supporting structure <b>75</b> facilitates the manufacture of the etched diaphragm <b>20</b> and/or its connection to the housing <b>45</b>.

0051<figref idref="DRAWINGS">FIGS. 4</figref><i>a</i>, <b>4</b><i>b</i>, <b>4</b><i>c</i>, and <b>4</b><i>d </i>show aspects of an optional head <b>50</b> analogous to the optional head shown in <figref idref="DRAWINGS">FIGS. 2</figref><i>a </i>and <b>2</b><i>b</i>. <figref idref="DRAWINGS">FIG. 4</figref><i>a </i>shows the optional head surrounding the housing <b>45</b>, which in turn surrounds the one or more transmitting optical fibers <b>70</b> and one or more receiving optical fibers <b>80</b>. Optionally, a fastener <b>52</b> (e.g., epoxy, acrylate adhesive) is used to fix the position and/or orientation of the optical fibers <b>70</b>, <b>80</b> within the housing <b>45</b>. <figref idref="DRAWINGS">FIG. 4</figref><i>b </i>shows an exemplary cross-section of the transmitting optical fiber <b>70</b> and the receiving optical fibers <b>80</b>. <figref idref="DRAWINGS">FIG. 4</figref><i>c </i>shows an illustrative cross-section of a protective layer <b>40</b>. <figref idref="DRAWINGS">FIG. 4</figref><i>d </i>shows an illustrative cross-section of a head <b>50</b>.

0052For the purpose of this discussion, a fiber optic sensor according to the instant invention having one transmitting optical fiber <b>70</b> and six receiving optical fibers <b>80</b> is called a 7 fiber probe. In an embodiment of a 7 fiber probe, light is coupled from a light source, e.g., a LED, into a transmitting optical fiber <b>70</b>, e.g., a multimode fiber, propagates to the well polished fiber end, is emitted from the fiber end, and is reflected back by an etched diaphragm <b>20</b> closely located to the fiber end. Part of the reflected light is coupled into the transmitting optical fiber <b>70</b>, and part of the reflected light is coupled into a bundle of six receiving optical fibers <b>80</b> and is detected by a photodetector. Vibration of the diaphragm center due to incoming sound pressure will modulate the detected light power.

0053A description of various performance characteristics of an embodiment of the 1 fiber probe and of an embodiment of the 7 fiber probe follows. It should be understood that all materials, dimensions, component models or manufacturers, connection means, steps, and masks are all given by way of non-limiting example to simplify examination and comparison of two illustrative embodiments of the instant invention.

0054For the purposes of this examination and comparison, embodiments of the 1 and 7 fiber probes use a multimode fiber having a 200 μm glass core, a 230 μm plastic (HCS, Spectran Specialty Optics) clad, a 500 μm Tefzel coating, and a 0.37 numerical aperture. The light source used in this work is an Optek OPF370A LED emitting light at 850 nm wavelength and the detector is a silicon PIN. Another optical component utilized in the 1 fiber probe is a 50/50 (at 850 nm), low loss, fiber coupler manufactured by Gould Electronics. The 1 fiber probe housing is a stainless steel tubing of 902 μm o.d and 584 μm i.d. The fiber was inserted in the tubing with its coating and epoxy was applied on the fiber. After the epoxy was cured, the fiber was cut closely to the tubing end and was polished very well.

0055The displacement sensitivity of the 1 fiber probe was studied by mounting it on a micrometer translator which could be displaced manually against a mirror mounted on a piezoelectric transducer (PZT-4 cylinder of 2″o.d and 3″ length) which could be vibrated electrically. First, the 1 fiber probe displacement sensitivity was achieved by displacing manually the probe against the mirror in steps of 25.4 μm using the micrometer translator. The results of this test are shown in <figref idref="DRAWINGS">FIG. 5</figref> where the power of the reflected light coupled into the same fiber is plotted vs. the probe-mirror distance. As can be seen from <figref idref="DRAWINGS">FIG. 5</figref>, maximum displacement sensitivity is achieved in the 0–150 μm probe-mirror distance, for this embodiment of the 1 fiber probe.

0056Then, the 1 fiber probe was dynamically displaced against the mirror by vibrating the PZT transducer electrically. The displacement amplitude of the vibrating mirror was obtained from the output of a small reference accelerometer (Endevco 2250A) mounted close to the mirror. The results of this test are shown in <figref idref="DRAWINGS">FIG. 6</figref> where the ac displacement sensitivity is shown as a function of the probe-mirror distance. As can be seen from <figref idref="DRAWINGS">FIG. 6</figref>, the sensitivity is maximum and constant in the 0–150 μm region, in agreement with the dc displacement results of <figref idref="DRAWINGS">FIG. 5</figref>. These results determine that the optimum operating distance of this embodiment of a 1 fiber probe end from the reflecting surface is about 60 μm and the optimum operating region for this embodiment of a 1 fiber probe is 0–120 μm.

0057In an embodiment of a 7 fiber probe, the seven fibers were inserted in a stainless steel tubing (1.270 mm o.d and 838 μm i.d) by first stripping their coating using a stripper having a 305 μm diameter blade hole. Then, epoxy was applied on the seven fibers, which were forced to form a symmetric bundle close to the tubing end with the transmitting fiber at the center, for example, as shown in <figref idref="DRAWINGS">FIG. 3</figref><i>b</i>. After the epoxy was cured, the fiber bundle was cut closely to the tubing end and the probe was polished.

0058The dc displacement sensitivity of this embodiment of a 7 fiber probe, which was studied in a similar way to that of the embodiment of a 1 fiber probe, is shown in <figref idref="DRAWINGS">FIG. 7</figref> where the reflected light power coupled into the <b>6</b> receiving fibers is plotted vs. probe-mirror distance. As shown in <figref idref="DRAWINGS">FIG. 7</figref>, maximum displacement sensitivity for this embodiment of a 7 fiber probe is achieved for 180–250 μm probe-mirror distances and is: <br />dc displacement sensitivity=9.38×10⁻¹¹W/A,  (1)<br /> where A=10⁻⁸cm. As can be seen from <figref idref="DRAWINGS">FIGS. 5 and 7</figref>, the region of maximum sensitivity of the embodiment of a 7 fiber probe is different from that of the embodiment of a 1 fiber probe, where the maximum sensitivity region starts from zero distance. Another difference is that the embodiment of a 7 fiber probe showed significantly higher light power detection due primarily to the fiber coupler used in the embodiment of a 1 fiber probe, which reduces the power by at least 50%.

0059The ac displacement sensitivity of the embodiment of a 7 fiber probe, which was obtained in a similar way to that of the embodiment of a 1 fiber probe, is shown in <figref idref="DRAWINGS">FIG. 8</figref> where the displacement sensitivity is plotted as a function of the probe-mirror distance. As can be seen from <figref idref="DRAWINGS">FIG. 8</figref>, maximum displacement sensitivity for this embodiment of a 7 fiber probe is achieved in the probe-mirror distance range 180–250 μm, in agreement with the dc displacement results of <figref idref="DRAWINGS">FIG. 7</figref>. This maximum displacement sensitivity range determines the optimum probe-reflecting surface distance to be about 220 μm for this embodiment of a 7 fiber probe. From <figref idref="DRAWINGS">FIGS. 8 and 6</figref> it can be also seen that the maximum ac displacement sensitivity of the embodiment of a 7 fiber probe is 13 dB higher than that of the embodiment of a 1 fiber probe. This significant sensitivity difference and the high cost of the multimode coupler required in the 1 fiber probe are advantages of using the embodiment of a 7 fiber probe instead of the 1 fiber probe, even though the latter sensor uses only one fiber instead of seven. This fiber difference can be significant, however, for remote sensing where long fiber lengths are required.

0060From <figref idref="DRAWINGS">FIG. 8</figref>, the displacement sensitivity of the embodiment of a 7 fiber probe was calculated from the signals of the probe and the reference accelerometer and was found to be: <br />Displacement sensitivity=6.35×10^{31 11}W/A  (2)<br /> This displacement sensitivity is slightly less than the sensitivity 9.38×10⁻¹¹W/A (Eq. 1) calculated from the dc displacement test. This shows that the displacement sensitivity is approximately the same from the displacements used in the dc test (˜1″) down to the minimum detectable displacement (Eq. 3 below). This dynamic range is about eight orders of magnitude.

0061The minimum detectable displacement was obtained by using a OPF 370A Optek LED driven at 100 mA current by a LDX-3620 Lightwave Technology power supply (in the battery operation). The detector was a TIA-500 Terahertz Technologies Inc. PIN detector, the output of which was stored into a 3582A HP spectrum analyzer. The fiber optic probe signal obtained with a known mirror displacement was compared to the noise which was the signal obtained with the mirror stationary. From that comparison, the minimum detectable displacement was found to be: <br />Minimum detectable displacement=1.1 A/(Hz)^1/2  (3)<br /> For a good PIN detector, which can detect a fraction of 1 pW ac signal, the minimum detectable displacement, limited only by the detector noise, is: <br />Minimum detectable displacement≧0.01A.

0062The instant fiber optic sensor has a thin etched diaphragm, as described above. These thin etched diaphragms can provide high acoustic sensitivities with relatively small diameters. Such etched diaphragms can be micromachined, or planar processed, reproducibly. An embodiment of a method to construct a silicon, etched diaphragm, for example, is described as follows. The starting material is a commercially available SOI (silicon on insulator) wafer. This wafer has a thin (e.g., 0.1 μm–2 μm) single crystalline silicon layer, a thin silica (sacrificing) layer, and a single crystalline silicon substrate (step I). On the silicon substrate a photoresist polymer is applied in the spin photoresist (step II). Then, using an appropriate photolithography mask, the wafer is exposed to light (step IV). After development, the desired pattern is formed (step IV). In the next reactive ion etching step, the substrate is exposed in a reactive ion beam, which etches away the silicon along the direction of propagation of the ions (step V). After cleaning the photoresist polymer in acetone (step VI), the wafer is dipped in HF in order to remove the silica layer (step VII). Then, the individual, silicon, etched diaphragms are formed by breaking the thin silicon layer (step VII). In this illustrative fabrication process, many silicon diaphragms can be made using the same mask. Using this micromachining process, many etched diaphragms can be formed in a highly reproducible, very stable, and inexpensive way.

0063For the purpose of determining acoustic sensitivity of an embodiment of a fiber optic sensor having a silicon, etched diaphragm formed as above, the etched diaphragm can be considered to be a semi-clamped circular plate, i.e., a plate halfway between a clamped and a simply supported one. The deflection of the center D of such an etched diaphragm when pressure p is applied is given by the following formula: <br /><i>D</i>=3<i>*p*R</i>⁴<i>*B</i>/(16<i>*E*h</i>³)  (6)<br /> where: <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0000"><ul id="ul0002" list-style="none"><li id="ul0002-0001" num="0064">B=B<b>1</b>=1−v²; clamped plate</li><li id="ul0002-0002" num="0065">B=B<b>2</b>=(5+v)/(1+v); simply supported plate <br /> In Eq. 6, R, h, E, and v are the radius, thickness, the Young's modulus, and the Poisson's ratio of the diaphragm, respectively. For this embodiment of a silicon, etched diaphragm: <br /><i>B</i>=(<i>B</i>1+<i>B</i>2)/2  (7)<br /> For this embodiment of a silicon, etched diaphragm: E=190*10¹⁰dynes/cm²; v=0.18. </li></ul></li></ul>

0066For a diaphragm with h=1 μm and R=1 mm: <ul id="ul0003" list-style="none"><li id="ul0003-0001" num="0000"><ul id="ul0004" list-style="none"><li id="ul0004-0001" num="0067">D<b>1</b>=5.968*10⁻⁷m/Pa; clamped plate</li><li id="ul0004-0002" num="0068">D<b>2</b>=2.620*10⁻⁷m/Pa; simply supported plate <br /> For this embodiment of a silicon, etched diaphragm: <br /><i>D=</i>(<i>D</i>1+<i>D</i>2)/2=1.608*10⁻⁷m/Pa<br /> Using the minimum detectable displacement given in Eq. 3, the minimum detectable pressure for this embodiment of a silicon, etched diaphragm is: <br /><i>P</i>_min=0.68 mPa/(Hz)^1/2  (8)<br /> This is an excellent minimum detectable pressure for such a pressure sensor having a cross-sectional width of around 1 mm. </li></ul></li></ul>

0069The bandwidth of a fiber optic sensor having a silicon, etched diaphragm as constructed above is determined as follows. The resonant frequency Fr of this embodiment of a micromachined silicon, etched diaphragm can be found from the following equation: <br /><i>Fr=</i>(½π)(<i>c/R</i>²)(<i>A/k</i>)^1/2  (9)<br /> where: A=E*h³*(1/(12*(1−v²)) and k=ρ*h; (ρ=2.3 gm/cm³, the silicon density).

0070In Eq. 9, <ul id="ul0005" list-style="none"><li id="ul0005-0001" num="0000"><ul id="ul0006" list-style="none"><li id="ul0006-0001" num="0071">c=10.2; clamped plate</li><li id="ul0006-0002" num="0072">c=5 simply supported plate.</li></ul></li></ul>

0073For this embodiment of a silicon, etched diaphragm: <br />i c=(10.5+5)/2  (10)<br /> For this embodiment of a silicon, etched diaphragm of radius R=1 mm and h=1 μm: <br />Fr=12.9 kHz  (11)

0074By way of comparison, the detectable pressure given by Eq. 8 and the bandwidth determined by Eq. 11 are very close to the requirements of a standard hearing-aid microphone: P_min=0.60 mPa/(Hz)^1/2, bandwidth: 100 Hz–14 kHz.

0075Due to the high degree of control in a planar processing technique, such as the micromachined diaphragm fabrication described above, fiber optic sensors according to the instant invention have highly reproducible and stable characteristics, such as sensitivity, phase, and bandwidth. Such small, light, reproducible, stable and inexpensive fiber optic pressure sensors are suitable in a variety of systems, such as microphone arrays. For example, microphone arrays can be used for more accurate measurements and monitoring of sound fields than is possible with a single microphone. Such an array is useful, for example, in condition monitoring of machinery to alert users of possible impending failures.

0076Obviously, many modifications and variations of the present invention are possible in light of the above teachings without departing from the true scope and spirit of the invention. It is therefore to be understood that the scope of the invention should be determined by referring to the following appended claims.